78 C. R. KEYES INTERMONT PLAINS OF THE ARID REGION 



In commenting on arid old age, Davis makes the statement that 



"It is little wonder that an understanding of the possible development ^r 

 rock-floored deserts of this kind, independent of baselevel, was not reached 

 inductively in western America ; for there has been so much disturbance in 

 way of fractures and uplift in that region during Mesozoic, Tertiary, and 

 Quaternary times that the attainment of old age has not been permitted ; but 

 that the problem was not solved deductively by the present generation of 

 American physiographers before it was encountered and solved by others in 

 Africa serves to show how insufficient still is the use of the deductive method 

 among us." 



There are apparently two very good reasons why American geologists 

 did not reach a solution of the problem before it had been arrived at else- 

 where. Deductive reasoning in geology is not always a very safe method 

 without more or less of an inductive foiindation. The great Colorado 

 plateau, containing more than 300,000 square miles of area and situated 

 more than a mile above sealevel, is mainly an even plain Avith a rock- 

 floor but thinly veneered by waste; yet these facts never excited the 

 notion of planation without the aid of water erosion. Moreover, Powell's 

 grand deduction relating to this very same arid region was something 

 very different from that proposed for the South African tableland. It 

 gave us the conception of the baselevel of erosion which, as it now seems, 

 was not entirely true of the arid region to which it was intended to apply, 

 though it aptly fitted the humid lands of the earth far removed from the 

 place of its birth. The philosopher Emerson somewhere says that when 

 we go to Europe we see only what we take with us. With etjual truth it 

 may be stated that when we go on geological travels we are often too 

 prone not to leave behind us some of our old notions. 



In the second place, scientists have only briefly visited the arid regions ; 

 of these, few have ever lived there for any length of time and have be- 

 come intimately acquainted with the peculiarities of the various geologic 

 processes working under new conditions. As a result, most of these 

 visitors have interpreted their olDservations more in accordance with the 

 better known conditions of a humid climate than of the less familiar 

 conditions of a dry climate. 



SHEETFLOOD EROSION 



McGee's graphic description^^ of sheetflood erosion has never received 

 the recognition that it jnstly merits from writers on the arid regions. 

 The remarkable phenomenon of the sheetflood is seldom observed to ad- 

 vantage except l)y dwellers within the dry belts of the earth. In these 





"Bulletin of the Geological Society of America, vol. S, 1897, pp. 87-112. 



